Most through-hole precision resistors are available in matched sets with specified ratio tolerance and tracking TCR, providing the best available precision. Options for the precision through-hole resistor approach range from semi-precision devices to ultra-precision devices capable of equalling the performance of costly metal foil technology using advanced metal film techniques.
The figure of most value to designers is the maximum total error in resistance value at the end of product life, or before scheduled re-calibration if this is applicable. Known as the total excursion, it is calculated from the root of the sum of the squares (RSS) of applicable, statistically independent short-term and long-term factors. Applying this calculation to an ultra-precision resistor demonstrates an order-of-magnitude improvement in total excursion compared with standard precision resistors.
Alternatively, where space is a major consideration, surface-mount device (SMD) thin-film products with multiple elements can provide high precision in a compact single-component solution.The precision SMD offering includes conventional thin-film chip resistors using nichrome elements. When evaluating the long-term stability of resistors, designers should consider several environmental tests. Some of these are early-life factors, such as exposure to solder heat. Others like TCR are reversible. Nevertheless, most are long-term factors. Best practice is to design based only on the figure that most closely reflects operating conditions.
Shelf life metrics apply where loading is negligible and the environment is benign, but the load figure should be applied where power dissipation is the main factor. For humid environments, designers should focus on measuring the long-term damp heat figure. In all these tests, most of the value change happens within the period of the test, as the value will tend to stabilize.
